A detailed knowledge of the processes involved in cellular determination is essential for understanding restrictions in cellular fate during development. Our specific goal has been to understand the determination of germ cells by the ooplasmic germ plasm, as a model system. In addition, we want to describe at the ultratructural level normal gastrulation in Drosophila and then to determine the nature of the defect produced by mutations affecting early development. Finally, we want to understand the mechanisms of vitellogenesis during oogenesis and yolk utilization during embryogenesis. Specifically, during the current year we intend to complete our fate-mapping of the wild-type embryo and of mutants affecting gastrulation, to characterize in detail the events occurring during oocyte activation in Drosophila, and to analyze cytological and molecularly ovary-specific vitellogenin synthesis. During early development, cells first form and then move to take on different cellular arrangements. The changes in cell surface will be analyzed by EM and immunological approaches. A number of maternal-effect mutations appear to affect these cellular movements, and these will be analyzed. A detailed molecular and immunological analysis of the early embryonic cell surfaces will be undertaken in order to provide the basis for interpreting mutant embryos. By means of experimental embroyology, our knowledge of the state of determination of cells will be determined and applied to mutants affecting gastrulation. Thus, I believe that a coordinated, structural, molecular, and genetic analysis of the Drosophila embryo is possible.